1. Field of the Invention
The invention relates to a semiconductor device including a bipolar transistor having a SiGe base epitaxial layer, and further to a method of fabricating the same.
2. Description of the Related Art
In order to improve a high-frequency characteristic in a transistor, there has been suggested a self-aligned selectively grown SiGe base (SSSB) bipolar transistor including an epitaxial layer made of SiGe alloy as a base layer.
FIGS. 1A to 1C are cross-sectional views of a base region of such a SSSB bipolar transistor, illustrating respective steps of a method of fabricating the same. As illustrated in FIG. 1A, a silicon dioxide film 2 and a polysilicon film 3 are successively formed on a monocrystal silicon substrate 1 acting as a collector. Then, the polysilicon layer 3 acting as a base region is etched to thereby form a second opening 3a therethrough. Thereafter, a silicon nitride film 4 is formed over an exposed surface of the polysilicon layer 3, namely, an upper surface of the polysilicon layer 3 and an inner surface of the second opening 3a.
Then, the silicon dioxide film 2 is wet-etched through the second opening 3a to thereby remove a portion of the silicon dioxide film 2 at a base region. Thus, the silicon dioxide film is formed with a first opening 2a which is in connection with the second opening 3a. By wet-etching the silicon dioxide film 2, the silicon dioxide film 2 is side-etched, resulting in that tunnel portions 2b are formed around the first opening 2a.
Then, as illustrated in FIG. 1B, a SiGe epitaxial film 5 acting as a base layer is selectively grown on a surface of the monocrystal silicon substrate 1, exposed to the second and first openings 3a and 2a, and the tunnel portions 2b.
The growth of the SiGe epitaxial film 5 is discussed in F. Sato et al., "A Super Self-Aligned Selectively Grown SiGe Base (SSSB) Bipolar Transistor Fabricated by Cold-Wall Type UHV/CVD Technology", IEEE Transactions on Electron Devices, Vol. 41, No. 8, pp. 1373-1378, August 1994, for instance. In the suggested method, disilane and germanium are used as a process gas for growth, and chlorine is used as an etching gas for selective growth.
In addition, an influence of chlorine gas on SiGe epitaxial layer growth is discussed in T. Aoyama et al., "Cl.sub.2 influence on Si.sub.1-x Ge.sub.x base epitaxial layer growth for high speed bipolar transistor", Extended Abstracts of the 1997 International Conference on Solid State Devices and Materials, pp. 528-529.
Referring back to FIG. 1B, the SiGe epitaxial film 5 is formed by means of a cold-wall type UHV-CVD apparatus wherein a base pressure is equal to or smaller than 1.5.times.10.sup.-9 Torr, disilane at 3 sccm and germanium at 2 sccm are used as a growth gas, chlorine at 0.03 sccm is used as an etching gas, and a growth temperature is set at 605 degrees centigrade.
Thus, the SiGe epitaxial layer 5 is formed on a surface of the monocrystal silicon substrate 1 in both the first opening 2a and the tunnel portions 2b. At the same time, a SiGe polysilicon film 6 is formed on a lower surface of the polysilicon film 3 exposed to the tunnel portions 2b.
Thus, the SiGe epitaxial film 5 grows upwardly, and the SiGe polysilicon film 6 grows downwardly both in the tunnel portions 2b until the SiGe films 5 and 6 come to contact with each other, as illustrated in FIG. 1C. Thus, a base layer consisting of the SiGe epitaxial film 5 makes electrical contact with the SiGe polysilicon film 6 making electrical contact with the polysilicon film 3 acting as a base electrode layer. Though not illustrated, an emitter layer and an emitter electrode are formed on a surface of the thus formed base layer 5 through the first opening 2a of the silicon dioxide film 2.
A bipolar transistor having such a structure as mentioned above can have a maximum cut-off frequency of 15 GHz.
In a conventional transistor as having been explained with reference to FIGS. 1A to 1C, since the SiGe epitaxial film 5 has a much greater growth rate than that of the SiGe polysilicon film 6, the SiGe epitaxial film 5 has a greater thickness than that of the SiGe polysilicon film 6 when they come to contact with each other, as illustrated in FIG. 1C. Hence, it is quite difficult or almost impossible to form the SiGe epitaxial film 5 to have a small thickness.
In particular, in accordance with the above-mentioned conventional method of epitaxial growth, the SiGe epitaxial film 5 occupies about 80% of a height of the tunnel portions 2b.
A time for electrons to run across a base layer can be made shorter, if the base layer has a smaller thickness. Namely, a base layer having a smaller thickness ensures a higher operation rate of a bipolar transistor. However, as mentioned so far, since it is quite difficult or almost impossible to form the SiGe epitaxial layer 5 thinner, there is paused a problem of difficulty in increasing an operation rate of a bipolar transistor by virtue of a thinner base layer.
In order to form the SiGe epitaxial layer or base layer 5 thinner, the silicon dioxide film 2 may be formed thinner to thereby decrease a height of the tunnel portions 2b. However, if the silicon dioxide film 2 is formed thinner, a parasitic capacitor formed between the monocrystal silicon substrate 1 acting as a collector and the polysilicon film or base electrode layer 3 would be increased. Thus, it would be necessary for the silicon dioxide film 2 to have a thickness of about 1000 angstroms at smallest. This means that the base layer 5 illustrated in FIGS. 1A to 1C would have a thickness of 800 angstroms at smallest, which prevents a bipolar transistor from operating at a higher rate.